In pipelines for long-distance conveyance of natural gas, crude oil, and the like, efforts have focused on improvement of conveyance efficiency through increasing running pressure. In order to enable a pipeline to withstand an increase in running pressure, a conceivable method is to increase the wall thickness of a conventional strength grade steel used for the pipe. However, this method leads to a reduction in efficiency of welding at the work site and a reduction in pipeline construction efficiency due to an increase in structural weight. Therefore, there has been increasing demand for limiting an increase in the wall thickness of the steel pipe through enhancement of the strength of steel products used for the pipe. As one measure to meet this demand, the American Petroleum Institute (API) has recently standardized X80 grade steel, and this steel has been put into practical use. The code "X80" represents a yield strength (YS) of not less than 80 ksi (approximately 551 MPa).
Further, there have been proposed several methods of manufacturing high-strength steel of X100 or X120 grade based on the technique of manufacturing X80 grade steel. Specifically, there have been proposed X100 through X120 grade steel whose strength is attained by making use of Cu precipitation hardening and a method of manufacturing the same (Japanese Patent Application Laid-Open (kokai) Nos. 8-104922, 8-209287, and 8-209288), as well as steel having an increase Mn content and a method of manufacturing the same (Japanese Patent Application Laid-Open (kokai) Nos. 8-209290 and 8-209291).
The former steel products manufactured through utilization of precipitation hardening surely have excellent field weldability and high base metal strength since hardness decreases at the heat affected zone of a welded joint. However, due to Cu precipitates dispersed within matrix, the arrestability of brittle fracture propagation (hereinafter referred to as "arrestability") is not sufficiently imparted. The arrestability is a property required of steel products in order to prevent a disastrous incident in which a welded steel structure would suddenly collapse due to brittle fracture.
Generally, the design of a welded steel structure takes account of the presence of defects of a certain degree in welded joints. Even when a brittle crack initiates from a defect present in a welded joint, if the base metal can arrest the propagation of the brittle crack, a disastrous incident could be prevented. Accordingly, for an large welded steel structure, welded joints must have a required anti-crack-initiation property (hereinafter referred to as "initiation property"), and the base metal must have required arrestability. Of course, in some cases, initiation property must be required for the base metal. Initiation property and arrestability are neither independent of nor unrelated to each other. For example, in the case in which hardening is induced by coherent precipitation of precipitates, both properties are impaired. Another factor--for example, refinement of microstructure--induces a great effect of improving initiation property, but merely a small (not zero) effect of improving arrestability. In discussing these two properties, it must be noted that a certain impact test provides a test result reflecting the two properties. The Charpy impact test provides a test result reflecting these two properties, but is said to reflect initiation property to a greater extent. In order to obtain a test result reflecting only arrestability, there must be employed DWTT or a double tension test, which will be described later in the EXAMPLES section, or a like test. Such tests use a relatively large test piece in which a portion where a brittle crack initiates and a portion where a brittle crack is arrested are separate from each other. Historically, these two properties have not been differentiated from each other, and a property obtained by the Charpy impact test or the like has been referred to as "toughness." Even at present, normally, so-called toughness includes arrestability and initiation property. Herein, unless otherwise specified, toughness refers to both arrestability and initiation property.
High-Mn-content steel disclosed in Japanese Patent Application Laid-Open (kokai) No. 8-209290 can assume required hardenability through containment of a large amount of Mn, which are relatively inexpensive, thereby reducing the use of Ni and Mo, which are expensive alloy elements. However, when the manganese content is increased and the nickel content is decreased, a welded joint will fail to assume the required initiation property, and the base metal will fail to assume required arrestability. A steel product which, as a base metal, has relatively low arrestability is not applicable to an important welded steel structure, and thus applications thereof are limited.
"Properties of welded joint" includes the toughness, particularly both "initiation property" and "strength," of a welded joint. A "welded joint" normally refers to both heat affected zone (including so-called "bond"; hereinafter abbreviated as HAZ) and weld metal. However, hereinafter, unless otherwise specified, a weld joint refers only to HAZ.
The above-mentioned line pipes are planned to be applied to high-pressure operation in the near future. In preparation for such applications, there has been demand for X120 grade steel products having required arrestability. X120 grade steel must have a YS of not less than 850 MPa. In this case, the TS of such steel becomes 900 MPa or higher. Steel products for line pipe use having such a high strength grade and sufficient arrestability have not yet been put into practical use.